feat(opencode): add conversation decoder and native resume projector #22

Outcome

Add first-class OpenCode transcript support so hosts can decode OpenCode export data into the neutral conversation model and project neutral conversations into importable OpenCode sessions.

Scope

• Decode { info, messages: [{ info, parts }] } OpenCode exports.
• Preserve user/assistant text, reasoning, images, and matched tool calls/results.
• Project provider-neutral conversations into OpenCode import data with deterministic valid IDs and explicit fidelity reporting.
• Export the new adapter APIs and cover round-trip/provider-pair behavior.

Integration

Agent Code will consume this through its provider-switch transcript adapter using the supported opencode export / opencode import CLI boundary.